1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, to a light emitting device in which an energy conversion efficiency can be improved by using an ultraviolet light emitting diode as an excitation source of a phosphor.
2. Discussion of the Background
A light emitting diode (LED) refers to a device for emitting light through recombination of minority carriers (electrons or holes) as a compound semiconductor having p-n junction structure. Since the light emitting diode has characteristics of low power consumption, long lifespan, installability in a narrow space, and strong resistance against vibration, the light emitting diode has been increasingly used as various illumination devices as well as parts of various information processing and communication applications and has been manufactured as various light emitting devices which are suitable to various applications. In recent years, white light emitting diodes in addition to single color light emitting devices, such as red, blue or green light emitting devices, have been placed on the market. As the white light emitting devices are applied to products for automobiles and illumination, it is expected that their demands will be rapidly increased.
In a representative method of implementing a white light emitting device, a yellow phosphor is arranged over a blue light emitting diode, so that blue light emitted from the blue light emitting diode and yellow light emitted from the phosphor which is excited by a portion of the blue light are color-mixed to implement white color. Although such a method of implementing white light provides a simple configuration and an excellent mass-producibility, there is a problem in that the shortages of green and red spectra may reduce color rendering.
In another representative method of implementing a white light emitting device, phosphors, which are excited by ultraviolet ray to emit blue, green and red lights, respectively, are arranged over a light emitting diode for emitting ultraviolet ray.
Since such a method uses ultraviolet ray, which is a higher excitation light source than blue light, to emit light having wavelengths from blue to red regions, there is an advantage in that the color rendering is higher. However, various phosphors, such as blue, green and red phosphors, are used, thereby increasing manufacturing costs.
Specifically, the red phosphor which is generally commercialized is sulfide based phosphor. In this case, there is a problem in that the light emitting device easily reacts with water vapor and carbon dioxide in the atmosphere during its operation and therefore the chemical characteristics of the phosphor is distorted. Since the byproduct of such a reaction, H2S gas, corrodes metal such as electrodes within the light emitting device, there is a problem in that the reliability of the light emitting diode may be reduced.
Further, there is another problem in that the red phosphor has a lower conversion efficiency for the light emitted under the excitation by the excitation energy than blue and green phosphors.